692 Transactions of the American Institute. 



between the actual and the theoretical diagrams serving as a mea- 

 sure of the perfection of the engine and valve mechanism. It is 

 impossible to construct an engine in which there shall be no loss 

 from friction of the steam in the pipes and passages, or from clear- 

 ance; but the engine in which these losses are least is unquestion- 

 ably the best. 



In the annexed diagrams the highest line represents the pressure 

 of steam in the boiler, and the lowest line that of atmospheric pres- 

 sure. The scale marked upon each diagram is the fraction of an 

 inch, which represents one pound of steam pressure in the vertical 

 lines of the diagram. The horizontal length of the diagram repre- 

 sents the length of stroke of the engine, plus an amount of space 

 at the end (exterior to the heavy outline), which is called the 

 " clearance," and represents, in the same scale as the stroke, the 

 amount of space included between the end of the cylinder and the 

 piston at the extreme of motion of the latter, and also the contents 

 of the passage ways. It will be seen that the length of stroke is 

 represented by no particular scale, but each of the divisions is one- 

 tenth of the full stroke. The heavy outline is the diagram formed 

 by the indicator, and represents truly the pressure in the cylinder 

 at each fraction of the motion of the piston. Where the line com- 

 mences to fall abruptly is the "point of cut-off," and shows what 

 portion of the stroke the steam is admitted. During the remainder 

 of the stroke the steam expands, reducing the pressure and forming 

 a curve called the "expansion curve." At, or just before, the end 

 of the stroke the steam is released, and "exhaust" commences. 

 The returning line shows, by its distance from the base line, the 

 amount of " back pressure," the exhaust closing a little before the 

 termination of the return stroke, thus confining the remaining steam, 

 compressing it, and forming a "cushion" to stop the momentum 

 of the piston, and prepare it for the return stroke. This is shown 

 by the rounding of the corner and the rising of the pressure at the 

 termmation of the stroke. 



The dotted outline represents the theoretical power of the 

 amount of steam exhausted from the cylinder in each instance, when 

 used in a cylinder of the same size, with no losses from friction in 

 the passages, back-pressure, or clearance. The proportion of tho 

 area of the actual to the theoretical diagram represents the relative 

 efficiency of the given engine, being in these diagrams as from 90 

 to 92| is to 100, which is believed to be better than is shown by the 

 carels taken from any other style of engines. 



